Beta-Plus Decay and Quarks

In beta-plus (β⁺) decay a proton in a proton-rich nucleus changes into a neutron. A positron and an electron neutrino are emitted:p → n + e⁺ + ν_eThe proton is uud and the neutron is udd, so at quark level one up quark changes into a down quark:u → d + e⁺ + ν_eCharge balances: +2/3 e = −1/3 e + (+1 e) + 0. Like β⁻ decay, this change of quark type is caused by the weak interaction.In the nucleus, the nucleon number A stays the same and the proton number Z goes down by 1. A free proton cannot decay this way: the neutron has a greater mass than the proton, so energy would not be conserved. Inside a proton-rich nucleus the extra energy comes from the change in binding energy of the nucleus.Worked example: fluorine-18, used in PET scanning, decays by β⁺ emission.¹⁸₉F → ¹⁸₈O + ⁰₊₁e + ν_eFluorine-18 has 9 protons and 9 neutrons, so it contains 2 × 9 + 9 = 27 up quarks. Oxygen-18 has 8 protons and 10 neutrons: 2 × 8 + 10 = 26 up quarks. Exactly one up quark has become a down quark.The positron is the antiparticle of the electron. It soon meets an electron and the two annihilate, producing gamma photons.